Highly oriented fluoropolymer films

ABSTRACT

A method of producing highly oriented multilayer films comprising coextruding films having at least one layer of a fluoropolymer, at least one layer of a polyolefin homopolymer or copolymer and an intermediate adhesive layer of a polyolefin having at least one functional moiety of an unsaturated carboxylic acid or anhydride thereof. With this structure the polyolefin layer allows the fluoropolymer layer to be stretched up to ten times its original length. Such a high orientation ratio for the fluoropolymer film increases the mechanical strength, toughness, and water vapor barrier properties of the film while using a thinner gauge fluoropolymer film. Coextrusion processing is done at higher temperatures, i.e. in the range of from at about 280° C. to about 400° C. These temperatures allow films to be produced in the absence of polymer degradation and film melt fracture.

This application claims the benefit of provisional application number60/020497 filed on Jun. 20, 1996, which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to oriented multilayer films. Moreparticularly, the invention pertains to coextruded or laminated filmshaving at least one layer of a fluoropolymer such aspoly(chlorotrifluoro ethylene) (PCTFE) homopolymer or copolymer, a layerof a polyolefin homopolymer or polyolefin containing copolymer and anintermediate adhesive layer of a polyolefin having at least onefunctional moiety of an unsaturated carboxylic acid and/or anhydridethereof

2. Description of the Prior Art

It is well known in the art to produce oriented polymeric films. See,e.g. U.S. Pat. No. 4,011,874. However, such films tend to expand in thedirection perpendicular to the direction of stretching.

It is also known in the art to produce single layer and multilayerfluoropolymer films. See, e.g. U.S. Pat. Nos. 4,677,017; 4,659,625 and5,139,878, all of which are incorporated herein by reference. However,fluoropolymers are difficult to orient due to their uniquecrystallization properties. More particularly, PCTFE is exceptionallydifficult to orient due to its extremely fast crystallization rate andthermally induced self-orientation. Its fast crystallization rateproduces a highly crystalline structure that hinders orientation andactually prevents further orientation beyond a certain point. Itsthermally induced self-orientation results in a film which, uponunconstrained heating, self extends in the machine or longitudinallystretched direction and shrinks in the transverse direction.

Most earlier attempts to stretch PCTFE films have failed either due toits high degree of film crystallinity, nonuniform crystallinity,self-orientation or a combination of these factors. Prior art studies ofthe orientation of PCTFE homopolymer report a limit of a three to fourtimes orientation or stretch ratio in either the machine direction (MD)or transverse direction (TD). For example, U.S. Pat. No. 4,544,721describes a substantially amorphous chlorotrifluoroethylene polymermonolayer film which is oriented at least 2.5 times its original length,but no more than five times in the MD. It also disclosed therein thatattempts to stretch crystalline PCTFE result in films that contain holesor tears, or which are uneven in thickness. Other known attempts tostretch PCTFE homopolymer more than five times its unstretched lengthresult in film fibrilation and ultimate breakage. See, e.g. U.S. Pat.No. 4,510,301 (orients film containing a copolymer of ethylene andchlorotrifluoroethylene).

It would be desirable to produce a much more highly oriented,dimensionally stable fluoropolymer film since as the higher the degreeof attainable orientation is increased, the properties of mechanicalstrength, toughness, and water vapor barrier capability aresignificantly improved without increasing the film gauge. It would alsobe desirable to produce a multilayered film structure which isdimensionally stable and uniform across its entire width.

SUMMARY OF THE INVENTION

The invention provides a multilayer film which comprises at least onefluoropolymer layer and at least one polyolefin layer comprising atleast one polyolefin homopolymer, polyolefin containing copolymer orblends thereof, attached to a surface of the fluoropolymer layer by anintermediate adhesive layer comprised of at least one polyolefin havingat least one functional moiety of an unsaturated carboxylic acid oranhydride thereof, which film has been uniaxially stretched at leastfive times in one linear direction, and wherein each of thefluoropolymer layer, adhesive layer and polyolefin layer have aviscosity of less than or equal to about 10,000 Pascal seconds at atemperature in the range of from about 280° C. to about 400° C.

The invention also provides a method of producing an oriented,multilayer film which comprises coextruding at least one layer of afluoropolymer, and at least one layer of a polyolefin homopolymer or apolyolefin containing copolymer attached to a surface of thefluoropolymer layer by a coextruded intermediate adhesive layer, whichintermediate adhesive layer is comprised of a polyolefin having at leastone functional moiety of an unsaturated carboxylic acid or anhydride,wherein said coextruding is conducted at a temperature of from about280° C. to about 400° C.; casting the film and then stretching the filmat least five times in either its longitudinal or transverse direction.

The invention further provides a method of producing an oriented,multilayer film which comprises laminating at least one layer of afluoropolymer to the surface of a layer of a polyolefin homopolymer or apolyolefin containing copolymer by an intermediate adhesive layer, whichintermediate adhesive layer is comprised of a polyolefin having at leastone functional moiety of an unsaturated carboxylic acid anhydride andthen stretching the film article at least five times in either itslongitudinal or transverse direction.

The invention still further provides an article which comprises athermoformed film of the above multilayered film.

The present invention achieves a highly oriented fluoropolymercontaining film by producing a multilayer structure by either acoextrusion or a lamination process. Without the additional layers inthe film structure, many fluoropolymers such as PCTFE can only bestretched to a maximum of five times its original length and usuallyonly three times stretching. With this structure, the polyolefin layerallows the fluoropolymer containing layer to be stretched more than fivetimes its original length, and usually up to ten times its originallength.

It has been further found that when fluoropolymer films are coextrudedwith polyolefins and adhered with the above intermediate adhesive layerat a temperature range of from about 280° C. to about 400° C., a stable,uniform film is produced.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

For purposes of this invention, the terms "orienting" and "stretching"shall be used interchangeably. As used herein, "copolymers" shallinclude polymers having two or more monomer components.

The fluoropolymer layer may be comprised of PCTFE homopolymers orcopolymers or blends thereof as are well known in the art and aredescribed in, for example, U.S. Pat. Nos. 4,510,301; 4,544,721; and5,139,878 which are incorporated herein by reference. Of these,particularly preferred fluoropolymers suitable to form multilayerbarrier films of the present invention include homopolymers andcopolymers of chlorotrifluoroethylene and copolymers ofethylene-chlorotrifluoroethylene. Such copolymers may contain up to 10%,and preferably up to 8% by weight of other comonomers such as vinylidinefluoride and tetrafluoroethylene. Most preferred arechlorotrifluoroethylene homopolymers and copolymers ofchlorotrifluoroethylene and vinylidine fluoride and/ortetrafluoroethylene. Such may also be purchased commercially as ACLON®resin from AlliedSignal Inc. of Morristown, N.J.

Adjacent to the fluoropolymer layer is an adhesive layer, also referredto in the art as a "tie" layer, between each film layer. In accordancewith the present invention, suitable adhesive polymers includes modifiedpolyolefin compositions having at least one functional moiety selectedfrom the group consisting of unsaturated polycarboxylic acids andanhydrides thereof Such unsaturated carboxylic acid and anhydridesinclude maleic acid and anhydride, fumaric acid and anhydride, crotonicacid and anhydride, citraconic acid and anhydride, itaconic acid ananhydride and the like. Of these, the most preferred is maleicanhydride. The modified polyolefins suitable for use in this inventioninclude compositions described in U.S. Pat. Nos. 3,481,910; 3,480,580;4,612,155 and 4,751,270 which are incorporated herein by reference. Thepreferred modified polyolefin composition comprises from about 0.001 andabout 10 weight percent of the functional moiety, based on the totalweight of the modified polyolefin. More preferably the functional moietycomprises from about 0.005 and about 5 weight percent, and mostpreferably from about 0.01 and about 2 weight percent. The modifiedpolyolefin composition may also contain up to about 40 weight percent ofthermoplastic elastomers and alkyl esters as described in U.S. Pat. No.5,139,878, which is incorporated herein by reference.

Adjacent the adhesive layer is a polyolefin layer comprised ofpoly(α-olefins) and copolymers and blends thereof, wherein the α-olefinmonomers have from about 2 to about 10 and preferably from about 2 toabout 6 carbon atoms. Non-limiting examples of polyolefins includepolyethylenes, including ultralow, low, linear low, medium, high andultrahigh density polyethylene; polypropylene; polybutylene;polybutene-1; polypentene-1; poly-3-methylbutene-1;poly-4-methylpentene-1; polyhexene; copolymers of polyolefins;copolymers of olefins and other polymers such as polyvinyl chloride,polystyrene and polyurethane, etc., and mixtures of these. Of these, thepreferred polyolefins are polyethylene and polypropylene withpolypropylene being most preferred.

Although each layer of the multilayer film structure may have adifferent thickness, the thickness of each of the fluoropolymer andpolyolefin layers of the films in the post-stretched multilayer filmsstructure is preferably from about 0.05 mils (1.3 μm) to about 100 mils(2540 μm), and more preferably from about 0.05 mils (1.3 μm) to about 50mils (1270 μm). The thickness of the post-stretched adhesive layer mayvary, but is generally in the range of from about 0.02 mils to about 12mils (305 μm), preferably from about 0.05 mils (1.3 μm) to about 1.0mils (25 μm), and most preferably from about 0.1 mils (25 μm) to about0.8 mils (20 μm). While such thicknesses are preferred as providing areadily flexible film, it is to be understood that other filmthicknesses may be produced to satisfy a particular need and yet fallwithin the scope of the present invention; such thicknesses which arecontemplated include plates, thick films, and sheets which are notreadily flexible at room temperature (approx. 20° C.).

In the preferred embodiment, each of the fluoropolymer layer, adhesivelayer and polyolefin layer have on average no embedded particles havinga diameter of greater than about 800 μm, no more than about 22 particleshaving a diameter of from about 400 to about 800 μm, no more than about215 particles having a diameter of from about 200 to about 400 μm and nomore than about 538 particles having a diameter of from about 100 toabout 200 μm per square meter of film and wherein each of thefluoropolymer layer, adhesive layer and polyolefin layer have on averageno more than about 0.36 embedded bubbles having a diameter of greaterthan about 3100 μm, no more than about 22 bubbles having a diameter offrom about 1500 to about 3100 μm, and no more than about 161 bubbleshaving a diameter of less than about 1500 μm per square meter of film.This allows for an extremely clear film having less likelihood ofbreaking or tearing. Each of the fluoropolymer layer, adhesive layer andpolyolefin layer materials have a melt viscosity of less than or equalto about 10,000, preferably from about 3,000 to about 10,000 Pascalseconds at a temperature in the range of from about 280° C. to about400° C., and preferably from about 285° C. to about 370° C.

These may be determined by using a Systronics Eagle Automatic InspectionSystem manufactured by Systronics, Inc.

The multilayer films of the present invention can have a variety ofstructures so long as there is an adhesive layer between each polymerlayer. A typical film structure includes a three-layer structure, whichcomprises a thermoplastic polyolefin layer, an adhesive layer and afluoropolymer layer. Another typical film structure is a five-layerstructure, which comprises a polyolefin layer, an adhesive layer, afluoropolymer layer, an adhesive layer and a polyolefin layer. These areonly two of many possible combinations of multilayer film structures,and any variation of the order and thickness of the layers of thefluoropolymer and polyolefin layer can be made.

The multilayer films of this invention may be produced by conventionalmethods useful in producing multilayer films, including coextrusion andextension lamination techniques. Suitable coextrusion techniques aredescribed in U.S. Pat. Nos. 5,139,878 and 4,677,017 except coextrusionin this invention is conducted at from about 280° C. to about 400° C.,preferably from about 285° C. to about 370° C. If coextrusion isperformed at a higher temperature, the film polymers tend to degradesignificantly and lose their film properties. If coextrusion is done ata lower temperature, the film has a non-uniform, hazy pattern indicativeof melt fracture. Coextrusion techniques include methods which includethe use of a feed block with a standard die, a multimanifold die such asa circular die, as well as a multimanifold die such as used in formingmultilayer films for forming flat cast films and cast sheets.

One advantage of coextruded films is the formation of a multilayer filmin a one process step by combining molten layers of each of the filmlayers of fluoropolymer, tie layer composition, and polyolefin, as wellas optionally more film layers, into a unitary film structure. In orderto produce a multilayer film by a coextrusion process, it is necessarythat the constituents used to form each of the individual films becompatible with the film extrusion process. The term "compatible" inthis respect means that the film-forming compositions used to form thefilms have melt properties which are sufficiently similar so as to allowcoextrusion. Melt properties of interest include, for example, meltingpoints, melt flow indices, apparent viscosity, as well as meltstability. It is important that such compatibility be present to assurethe production of a multilayer film having good adhesion and relativelyuniform thickness across the width of the film being produced. As isknown in the art, film-forming compositions which are not sufficientlycompatible to be useful in a coextrusion process frequently producefilms having poor interfacial lamination, poor physical properties aswell as poor appearance.

One skilled in the art can readily weigh the above-noted compatibilityin order to select polymers having desirable physical properties anddetermine the optimal combination of relative properties in adjacentlayers without undue experimentation. If a coextrusion process is used,it is important that the constituents used to form the multilayer filmbe compatible within a relatively close temperature range in order topermit extrusion through a common die. It has been found that thevariation of the quantity of the modified polyolefin within the tielayer composition provides an adhesive layer forming composition whichis of sufficiently high melt viscosity, especially in the preferredrange of compositions described above, to be particularly useful in acoextrusion process with the fluoropolymer film forming composition, andwith a film forming composition.

Alternatively, the multilayer films of the present invention can beproduced by lamination whereby a multilayer film structure is formedfrom pre-fabricated film plies. The basic methods used in filmlaminating techniques are fusion, wet combining, and heat reactivating.Fusion, which is a method of laminating two or more film plies usingheat and pressure without the use of other adhesives can only be usedwhere the films being laminated are comprised of polymers that readilyform interfacial adhesion. Wet combining and heat reactivating areutilized in laminating incompatible films using adhesive materials.

Typically, laminating is done by positioning the individual layers ofthe inventive film on one another under conditions of sufficient heatand pressure to cause the layers to combine into a unitary film.Typically the fluoropolymer, adhesive, and polyolefin layers arepositioned on one another, and the combination is passed through the nipof a pair of heated laminating rollers by techniques well known in theart such as those described in U.S. Pat. No. 3,355,347 which isincorporated herein by reference. Lamination heating may be done attemperatures ranging from about 120° C. to about 175° C., preferablyfrom about 150° C. to about 175° C. at pressures ranging from about 5psig (0.034 MPa) to about 100 psig (0.69 MPa) for from about 5 secondsto about 5 minutes, preferably from about 30 seconds to about 1 minute.

The multilayer film, whether comprising or three or more layerstructure, may be stretched or oriented in any desired direction usingmethods well known to those skilled in the art. Examples of such methodsinclude those set forth in U.S. Pat. No. 4,510,301. In such a stretchingoperation, the film may be stretched uniaxially in either the directioncoincident with the direction of movement of the film being withdrawnfrom the casting roller, also referred to in the art as the "machinedirection", or in as direction which is perpendicular to the machinedirection, and referred to in the art as the "transverse direction", orbiaxially in both the machine direction and the transverse direction.The multilayered film of the invention are particularly useful forforming thermoformed three dimensionally shaped articles such as blisterpackaging for pharmaceuticals. This may be done by forming the filmaround a suitable mold and heating in a method well known in the art.

We have unexpectedly found that the fluoropolymer films of the presentinvention have sufficient dimensional stability to be stretched at leastfive and preferably more than five times and more preferably from morethan five times to about ten times in either the machine direction orthe transverse direction or both.

Another noteworthy characteristic of the films of the present inventionis that they exhibit improved tensile modulus, mechanical strength, andthe most significantly of all, excellent barrier properties towards bothwater vapor and oxygen at 100% relative humidity after being stretchedfive or more times its original length uniaxially in either machinedirection or transverse direction.

Water vapor transmission rate (WVTR) may be via the procedure set forthin ASTM F1249. In the preferred embodiment, the multilayered filmaccording to this invention has a WVTR of from about 0.001 to about 0.05gm/100 in² /day per mil thickness of PCTFE, preferably from about 0.002to about 0.02 gm/100 in² /day per mil thickness of PCTFE, and morepreferably from about 0.002 to about 0.01 gm/100 in² /day per milthickness of PCTFE. For example, a three layered film having aPCTFE/adhesive layer/polyolefin layer structure which is oriented sixtimes its original length in the machine direction possesses a WVTR of0.0051 gm/100 in² /day per mil thickness of PCTFE which is 200% betterthan the unoriented equivalent sample (WVTR 0.017 gm/100 in² /day permil thickness) and almost 100% better than an equivalent film samplestretched only three times its original length (0.0098 gm/100 in² /dayper mil thickness.

Oxygen transmission rate (OTR) may be via the procedure of ASTM D-3985using an OX-TRAN 2/20 instrument manufactured by Modern Controls, Inc.,operated at 73° F., 90% RH. In the preferred embodiment, themultilayered film according to this invention has an OTR of from about0.1 to about 10 cc/100 in² /day per mil thickness of PCTFE, preferablyfrom about 0.5 to about 5 cc/100 in² /day per mil thickness of PCTFE,and more preferably from about 0.5 to about 3 cc/100 in² /day per milthickness of PCTFE. The following non-limiting examples serve toillustrate the invention.

EXAMPLES

The operation of the laboratory film stretcher employed in all of thefollowing examples is based on the movement of two draw bars at rightangles to each other upon hydraulically driven rods. These pairs of drawbars, to which the four edges of a film specimen are attached, form thetwo axes at right angles to each other along which a specimen isstretched in any desired stretch ratio. Films can be stretched in one orboth directions independently or in both directions simultaneously. Thestretching may be done at any selected constant rate adjustable from0.51 to 50.8 cm per second or at any constant force from zero to 11.3 kgper inch of edge before stretching. Nominal sample size beforestretching is 10 cm by 10 cm between grips for stretching under 4 timesoriginal size. For stretching between 4 times and 7 times original size,the sample size is 6 cm×6 cm. Specimens may be heated in a controlledmanner during the stretching cycle, similar to the commercial tenteroven. The following examples employed a constant stretch rate of 25.3 cmper second and a stretch temperature at 90°-100° C. with six secondspre-heating at a temperature within the same range.

Example 1

PCTFE homopolymer (HP)(390,000 M.W., density: 2.11 gm/cc; meltingtemperature: 211° C.; Zero Strength Test (ASTM D1430): 128, manufacturedby AlliedSignal Inc. under the tradename Aclon® HP 128), was heated forfour hours at 121° C. for drying, and then extruded through a 3.2 cm(11/4") diameter Killion single screw extruder (L/D=2411) equipped withthree heating zones and two adapters. The extruder temperature profilewas set at 277° C., 282° C., and 288° C. for the zones 1-3, and theadapters were maintained at 288° C. The melt temperature was measured at286° C. After passing the extrudate through a coextrusion film diemaintained at 282° C., it was cast on a roller maintained at 38° C.,followed by a cooling roller set at 38° C. The resultant film had athickness of 25 μm although other films with various thicknesses up to150 μm were also made. Immediately after casting, the films werestretched off-line in a T.M. Long laboratory stretcher set at 100° C.Cast film samples were cut to either 10 cm×10 cm or 6 cm×6 cm, dependingon the intended stretching ratio. These film samples were then loadedinto the laboratory stretcher equipped with grips along all four edges.After six seconds of preheating, the samples were then stretched to adesired stretch ratio, which was preset on the draw bar in the stretcherbefore the experiment. Films so obtained were then tested for variousmechanical and physical properties as illustrated in Tables 1 and 2.

In all attempts to stretch monolayer PCTFE homopolymer greater thanthree times its unstretched length, the film always fibrilated andultimately broke.

Example 2

A five layer laminate was coextruded using the PCTFE HP of Example 1, apoly(propylene) copolymer with polyethylene (melting temperature: 148°C.; melt flow rate MRF (ASTM D1238): 1.9 gm/10 min. at 230° C.; 3.2 %ethylene content, manufactured by Shell Chemical Co. under the tradename6E20), and a maleic anhydride modified polyolefin tie resin (density:0.88 gm/cc melt index: 1.0 gm/10 min. at 190° C., manufactured by MitsuiPetrochemical Industries, Ltd. under the tradename "Admer") to make thefollowing structure: poly(propylene)/tie resin/PCTFE HP/tieresin/poly(propylene).

Poly(propylene) copolymer was extruded through a 3.8 cm (11/2") diameterKillion single screw extruder (L/D=24/1) equipped with three heatingzones and two adapters. The extruder temperature profiles were set at238° C., 249° C., 260° C. for the zone 1-3 and the adapters weremaintained at 260° C. The melt temperature was 256° C. The maleicanhydride modified tie resin was extruded through a 3.2 cm (11/4")diameter Killion single screw extruder equipped with four heating zonesand two adapters. The extruder temperature profiles were set at 238° C.,249° C., 260° C., 266° C. for the zone 1-4 and the adapters weremaintained at 266° C. The resulting melt temperature was 263° C. Thefluoropolymer was extruded following the same procedures described inExample 1.

The five layer extrudate, after passing through a coextrusion multilayerfilm die maintained at 282° C., was then cast on a roller kept at 38°C., followed by a cooling roll set at 38° C. The resultant film had athickness of 25 μm although other films with various thicknesses up to233 μm were also made. Immediately after the casting the films werestretched according to the method set forth in Example 1. Thepost-stretching layer thickness of the PCTFE homopolymer layer is about38% of the total thickness, while the poly(propylene) layers and the tielayers consist of the remaining 62% of the total post-stretchingthickness. In order to make direct comparison in the test properties,the PCTFE homopolymer layer was then carefully separated from the otherlayers in the multilayer film without any distortion or dimensionalchange.

This example illustrates that the PCTFE film, when coextruded with thetie and polypropylene layers can be stretched more than five timesuniaxially in either machine direction (MD) or transverse direction (TD)with great ease without film breakage. The tensile modulus of the PCTFEfilm, when stretched more than five times its original length, issignificantly higher than its counterparts as shown in Table I. As shownin Table II, the WVTR for PCTFE films oriented six times their originallength in the MD was 0.0051 gm/10⁶ in² /day per mil thickness of PCTFE,which is 200% better than the unoriented sample and almost 100% betterthan the PCTFE film sample stretched only three times its originallength.

                  TABLE 1    ______________________________________    MECHANICAL PROPERTIES OF ORIENTED PCTFE    HOMOPOLYMER                         DEGREE OF          TENSILE MODULUS.sup.(1),                         CRYSTALLINITY.sup.(2)    SAMPLE          MPa(psi)       (%)           EXAMPLE    ______________________________________    Cast  1054.17 × 10.sup.6 (153,000)                         36.3    Mono-    layer    PCTFE    Cast  1067.95 × 10.sup.6 (155,000)                         38.0          2    Five-    layer    PCTFE    2X MD 1371.11 × 10.sup.6 (199,000)                         42.4          1 and 2    3X MD 1722.5 × 10.sup.6 (250,000)                         43.1          1 and 2    4X MD 1860.3 × 10.sup.6 (270,000)                         45.1          2    5X MD 2411.5 × 10.sup.6 (350,000)                         45.1          2    5X MD 2053.22 × 10.sup.6 (298,000)                         45.6          2    6X MD 2618.2 × 10.sup.6 (380,000)                         46.0          2    6X MD 2259.92 × 10.sup.6 (328,000)                         45.5          2    7X MD 2232.36 × 10.sup.6 (324,000)                         47.2          2    ______________________________________     .sup.(1) Tensile modulus in the direction stretched. For cast film     samples, tensile modulus was measured in the machine direction (MD) using     ASTM D882.     .sup.(2) Degree of crystallinity determined by Differential Scanning     Calorimetry (DSC). DSC scans were obtained on a TA 9900 Automated DSC     unit. A 10.0 ± 0.2 mg sample was crimped in an aluminum pan and heated     at a rate of 50° C./min. in an argon atmosphere. A DSC crystalline     index was calculated from the ratio of the corrected heat of melting,     ΔH.sub.m, and the heat of fusion, ΔH.sub.m = 43 J/g of a 100%     crystalline PCTFE.

                  TABLE 2    ______________________________________    BARRIER PROPERTIES OF ORIENTED PCTFE HOMOPOLYMER             WVTR.sup.(3), gm                          OTR.sup.(3), cc mil/100             mil/100 in.sup.2 /day at                          in.sup.2 /day at 73° F., 90%                                        EXAM-    SAMPLE   100° F., 100% RH                          RH            PLE    ______________________________________    Cast PCTFE             0.017        4.7           1 and 2    Homopolymer    (control)    2X TD    0.0114       3.8           1 and 2    3X TD    0.0098       3.2           1 and 2    6X TD    0.0051       2.7           2    ______________________________________     .sup.(3) Both WVTR and OTR were measured in a MOCON instrument according     to ASTM Test Method F1249, the results of which are based on per mil     thickness of PCTFE.

Thus it can be seen that the uniaxial orientability of PCTFE film can beincreased to attain seven times in either the MD or TD through itscoextrusion with polyolefins. As illustrated in Table I, not only thetensile modulus, but also the degree of crystallinity of the coextrudedPCTFE films significantly improved. The degree of crystallinity of allfilms ranged from 36% to 49%, a significant increase as one increasesthe orientation ratio. As illustrated in Table 2, the barrier propertiesimprove significantly as the stretch ratio increases. More specifically,the WVTR for a six-time MD oriented film sample, at (0.0051 gm/100 in²/day per mil thickness of PCTFE) was 30% lower than the WVTR for theunoriented control film (0.017 gm/100 in² /day per mil thickness ofPCTFE). This level of moisture barrier is the best ever known for athermoplastic material, approaching the barrier properties of the metaland glass, which are considered impermeable. The OTR was alsodramatically reduced by about 57%, from 4.7 cc/100 in² /day per milthickness of PCTFE for the unoriented film sample to 2.7 cc/100 in² /dayper mil thickness of PCTFE at 55° C. and 90% RH for the sample orientedsix times it original length.

What is claimed is:
 1. A method of producing an oriented, multilayerfilm which comprises coextruding at least one layer of a fluoropolymer,and at least one layer of a polyolefin homopolymer or a polyolefincontaining copolymer attached to a surface of the fluoropolymer layer bya coextruded intermediate adhesive layer, which intermediate adhesivelayer is comprised of a polyolefin having at least one functional moietyof an unsaturated carboxylic acid or anhydride, wherein said coextrudingis conducted at a temperature of from about 280° C. to about 400° C.;casting the film and then stretching the film at least five times ineither its longitudinal or transverse direction.
 2. The method of claim1 further comprising coextruding another layer of a polyolefinhomopolymer or a polyolefin containing copolymer to another surface ofthe fluoropolymer layer by another intermediate adhesive layer which iscomprised of a polyolefin having at least one functional moiety of anunsaturated carboxylic acid anhydride; wherein said coextruding isconducted at a temperature of from about 280° C. to about 400° C.
 3. Themethod of claim 1 comprising coextruding and attaching another layer ofa fluoropolymer to another surface of the layer of the polyolefinhomopolymer or polyolefin containing copolymer by another intermediateadhesive layer comprised of a polyolefin having at least one functionalmoiety of an unsaturated carboxylic acid or anhydride thereof; whereinsaid coextruding is conducted at a temperature of from about 280° C. toabout 400° C.
 4. The method of claim 1 wherein each of the fluoropolymerlayer, adhesive layer and polyolefin layer have on average per squaremeter of film no embedded particles having a diameter of greater thanabout 800 μm, no more than about 22 particles having a diameter of fromabout 400 μm to about 800 μm, no more than about 215 particles having adiameter of from about 200 μm to about 400 μm, and no more than about538 particles having a diameter of from about 100 μm to about 200 μm. 5.The method of claim 1 wherein each of the fluoropolymer layer, adhesivelayer and polyolefin layer have on average per square meter of film nomore than about 0.36 embedded bubbles having a diameter of greater thanabout 3100 μm, no more than about 22 bubbles having a diameter of fromabout 1500 μm to about 3100 μm, and no more than about 161 bubbleshaving a diameter of less than about 1500 μm.
 6. The method of claim 1wherein each of the fluoropolymer layer, adhesive layer and polyolefinlayer have a viscosity of from about 3,000 to about 10,000 Pascalseconds at a temperature in the range of from about 280° C. to about400° C.
 7. The method of claim 1 wherein the fluoropolymer is selectedfrom the group consisting of chlorotrifluoroethylene homopolymers,chlorotrifluoroethylene containing copolymers, and blends thereof. 8.The method of claim 1 wherein the fluoropolymer is apoly(chlorotrifluoro ethylene) homopolymer.
 9. The method of claim 1wherein the fluoropolymer is a poly(chlorotrifluoro ethylene) containingcopolymer.
 10. The method of claim 1 wherein the polyolefin layer isselected from the group consisting of poly(propylene), poly(ethylene),poly(butylene), copolymers containing a polyolefin and mixtures thereof.11. The method of claim 1 wherein the unsaturated carboxylic acidanhydride is maleic anhydride.
 12. The method of claim 1 wherein thefilm is uniaxially stretched in its longitudinal direction.
 13. Themethod of claim 1 wherein the film has been uniaxially stretched in itstransverse direction.
 14. The method of claim 1 wherein the film isuniaxially stretched from at least 5 times to about ten times.